hrm. good question. so, the names you see at
https://build.opensuse.org/repositories/filesystems ("15.6", "15.7", "16.0", "SLE_15_SP5", "SLE_15_SP6", etc. are just arbitrary strings defined in
https://build.opensuse.org/projects/filesystems/meta. What's actually interesting, is the projects those repos build on top of. So the filesystems "15.7" repo really is SLE 15 SP7, because that's what it builds on top of.
I don't know why 15.6 is set to build against Leap 15.6 Updates and 15.7 is set to build against SLE 15 SP7 Updates, but in practice it probably doesn't matter, because IIRC Leap 15.X and SLE 15 SPX have common sources. So filesystems 15.6 should work equally well on Leap 15.6 and SLE 15 SP6, and filesystems 15.7 should work equally well on Leap 15.7 and SLE 15 SP7 and so on.
Micro 6.1 is a bit different because it builds against the SLFO 1.1 (former ALP) repos. I don't think it really has an exactly matching SLE 15 SPx version.
This becomes unified again once we get to Leap 16 / SLE 16 / SL Micro 6.2 which AIUI somehow get back in sync again, all based on newer SLFO 1.2 sources. (the plan is to bump Harvester v1.8 to SL Micro 6.2).